See more from this Division: Topical Sessions
See more from this Session: Modes of Lithospheric Extension: Oceanic and Continental Core Complexes
Abstract:
Multiple lines of evidence document major footwall rotation. Thermochronologic data shows that prior to exhumation, a ~250-300°C difference existed across ~20 km of the footwall in the slip direction, implying 20-50° of footwall tilting (assuming geothermal gradients of 15-40°C/km). Variations in deformation mechanisms in footwall mylonites match this thermal gradient. Geologic relationships confirm major footwall tilting. An Oligocene dike swarm dips 40-50°SW throughout the northern footwall, indicating 40-50° of footwall tilt (assuming initially vertical dikes). Preliminary paleomagnetic data from these dikes (Platzman et al., 2006) supports similar footwall tilt. And finally, we interpret a contact in the easternmost footwall as a Cenozoic unconformity, above which Oligocene sediments dip 40-50° NE. Together, these results strongly support 40-50° of NE footwall tilting since the fault formed, indicating an initial fault dip of 50-60°. The lack of appropriate footwall structures and the presence of normal faults east of the core complex cause us to favor domino-style fault rotation over a rolling hinge model. These results suggest that not all core complexes represent a unique mode of extension but rather can be formed by major slip on an otherwise typical steeply dipping normal fault. It remains unclear how many other Cordilleran core complexes have experienced similar tilting.
See more from this Division: Topical Sessions
See more from this Session: Modes of Lithospheric Extension: Oceanic and Continental Core Complexes